There is a need for hollow ceramic spheroids with controlled porosity for use as fillers in certain concrete applications. One application for concrete containing such a filler is the concrete which surrounds pipe sections used for gas pipelines in the ocean. Such pipe sections comprise a metal core pipe covered with a concrete shell. Hollow, porous spheroids within the concrete would initially allow the pipeline to weigh less than such pipe without the hollow filler, but with the passage of time, water permeating into the porous hollow spheroids in the concrete would increase the weight of the pipeline, making it rest stably on the ocean bottom after it had been assembled and installed. For such applications, it is desirable to have hollow spheroids whose porosity and strength remain relatively stable so that the performance of the concrete around the pipeline is predictable.
Other potential uses for hollow fired spheroids are in the formation of low density syntactic bodies with high temperature strength. Such syntactic bodies may be useful as heat resistant insulation. Syntactic foams are also useful as flotation materials for off-shore drilling rigs and other water-borne vessels.
There are a number of patents on the subject of hollow, ceramic spheroids, sometimes called macrospheres to differentiate them over microspheres (macrospheres being generally larger than 500 micrometers in diameter while microspheres are generally less than that dimension). The Beck macrospheres of U.S. Pat. Nos. 4,111,713 and 4,393,901 are made by combining a binder material and a particulate parting agent (e.g. hollow glass microspheres). A void-forming agent is included in the binder and causes the formation of the hollow center of the Beck macrospheres during their manufacture.
There are also known processes for making spheroids by coating small globules or impregnating particles of some combustible material (e.g. a plastic resin or wood flour) with a ceramic material, decomposing the combustible organic, and firing the ceramic shell, see, e.g. U.S. Pat. Nos. 2,553,759; 4,039,480 and 3,792,136.